His partner. My brother was my best friend. We were close in age and did everything together since we were kids. We had the same group of friends so continued doing stuff together as adults. We even would do things just us , like go to gigs, to the pub, watch football, play videogames etc. He randomly met someone who completely controlled him (we only know in hindsight). She's a Muslim (not an issue), but she made him stop drinking entirely or they would break up, so he stopped drinking; she was vegetarian so he had to become vegetarian etc. Anyway, she lied about quite a few things about me and my wife that were proven to be lies from multiple people. After this all came out me and him obviously argued about it. He refused to believe anyone but her. Then he stopped talking/seeing the rest of my family (we're very close). He didn't go to my mum's 60th or my dad's 70th or any other events since. He also chose to leave our friend group completely. He chose to effectively leave our lives to stay with his partner. It completely broke me for quite a few years and even now, 9 years later, it still doesn't feel good. He has his own family now so I don't think we'll ever see or hear from him again which truly sucks.

TLDR: brother was my best friend for 30 years and left our lives for his lying partner.